Fix #133097: GP Vertex Weight Proximity wrong with influence VG

The modifier would write to all weights, not respecting if a vertex was
not part of the influence vertex group.
If the vertex is not part of the influence vertex group (its weight is
0.0), dont touch the existing weights (just continue happily).

Pull Request: https://projects.blender.org/blender/blender/pulls/133098

source/blender/modifiers/intern/MOD_grease_pencil_weight_proximity.cc

@@ -174,7 +174,7 @@ static void write_weights_for_drawing(const ModifierData &md,
   threading::parallel_for(positions.index_range(), 1024, [&](const IndexRange range) {
     for (const int point_i : range) {
       const float weight = vgroup_weights[point_i];
-      if (weight < 0.0f) {
+      if (weight <= 0.0f) {
         continue;
       }